The terminal phase of myogenesis includes specific alignment or "recognition" of myoblasts, perhaps followed by specific cell-cell adhesion, but certainly followed by fusion to form multinucleate myotubes. Cell surface proteins must play a significant role in, and perhaps provide all the specificity for, the critical prefusion and fusion events. I have proposed that each stage of myogenesis is characterized by a specific set of protein "receptors" on the cell surface which receive and transduce environmental signals, as well as participate in cell-cell recognition, alignment, and fusion. I have already identified a set of developmentally regulated myoblast surface proteins which could be involved in these processes. Several of these proteins have been partially purified. This proposal delineates methods to complete the analytical purification of the proteins, and further analyze the proteins, including: (1) production of antibodies to the purified proteins and utilization of the antibodies to purify milligram quantities of the proteins; (2) insertion of the purified proteins into biological membranes; (3) use of the antibodies to define the biological role of the proteins; (4) use of fluorescent antibodies to analyze the distribution of the proteins on the myoblast surface; (5) in situ chemical characterization of the regulated proteins including protein asymmetry in the membrane and "nearest neighbor" analyses. A major thrust of this proposal is the use of our non-fusing variants of a rat myoblast cell line to analyze pre-fusion and fusion events. These spontaneous variants will be used extensively in the analysis of the biological role of the purified proteins. Temperature-sensitive non-fusing variants will also be isolated, and used to examine both interdependence in the sequence of surface changes, and the regulation of some of those surface events.